The present invention relates to oximeter sensors, and in particular oximeter sensors with a heating element to improve perfusion.
Pulse oximetry is typically used to measure various blood characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, and the rate of blood pulsations corresponding to a heart rate of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient""s tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted or reflected light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Heaters have been used in sensors to improve the perfusion, or amount of blood, adjacent the sensor. This will thus improve the measurement since the light will encounter a larger volume of blood, giving a better signal-to-noise ratio for the oximeter reading.
U.S. Pat. No. 4,926,867 shows a piece of metal used as a heater in an oximeter sensor. A separate thermistor is used to measure the amount of heat so that the heater can be controlled to avoid burning the patient.
U.S. Pat. Nos. 5,299,570 and 4,890,619 both show ultrasonic elements being used for perfusion enhancement.
Because the normal human body core temperature is approximately 37xc2x0 C., and burning of tissue could take place for temperatures above approximately 42-43xc2x0 C., a tight range of control of the heating element is required. Another challenge is the heat gradient and delay time between the heating element and the temperature measuring element.
The present invention provides a method and apparatus for both heating a patient""s skin and for measuring the temperature using the same device, such as a thermistor. Thus, the thermistor generates controlled heat, and is not just used for sensing the temperature. In an oximetry sensor, the thermistor is located in the vicinity of the light emitter and photodetector to warm the optically-probed tissue region. As heat is dissipated, temperature changes are sensed as resistance changes according to Ohm""s law. Active thermal regulation by varying the amount of thermistor current and power can safeguard against burning the tissue while maximizing perfusion.
It has been shown recently that general warming of the tissue region increases the amount of blood perfused in the tissue. This increased perfusion substantially strengthens the pulse oximetry signal. Benefits include quick signal acquisition, increased accuracy, and greater tolerance to motion artifact.
In one embodiment, the thermistor is a positive temperature coefficient (PTC) thermistor rather than the more common, negative temperature coefficient (NTC) thermistor. The PTC provides a highly desirable safety feature as poor connections yield a perceived, higher-than-normal resistance indication. As a result, the actual thermistor temperature is regulated at a lower-than-expected temperature, avoiding the chance of burns.
Another advantage of the same thermistor being used for both generating heat and temperature measurement is that there is no thermal gradient between the heating element and the sensing element as in the prior art. This allows for a faster response time, which is critical for maintaining a tight temperature range.
The thermistor""s resistance can be conventionally determined either by a two-wire or a four-wire method. The four-wire method is typically used when the connections used in the two-wire method would have resistances that could significantly affect the measurement. In the four-wire method, one pair of wires is used to inject a known current through the thermistor, while the other pair is used to sense the voltage across the thermistor. This enables a highly accurate determination of the thermistor""s temperature.
In an alternate embodiment, a simple bridge circuit with a setpoint resistor may be used to automatically bias the thermistor at a particular resistance/temperature. Once the thermistor""s desired operating resistance is known from the factory, the appropriate value of the setpoint resistor can be employed in the circuit. This simple circuit could be integrated into the sensor itself or in the remote monitor.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.